Tires support a vehicle body. The tires are under a load. Thus, flexure occurs in the tire. The maximum load capacity of one tire in the supporting is represented as an index. As the index, a load index is known. The load index is defined in the JATMA standard. The load index is an index that represents the maximum weight that is allowed to be applied to a tire in a specified condition.
A truck runs in a state where the truck is loaded with a cargo. A truck runs in a state where the truck is loaded with a cargo that is approximately equivalent to the maximum loading capacity in some cases. In this case, the tire is under a load equivalent to the load index. Thus, in the tire, flexure is great in bead portions thereof. The great flexure causes strain. The strain tends to concentrate on a boundary between a carcass and an apex, the end of the apex, and a boundary between the carcass and a clinch. The concentration of the strain may cause damage such as loosening.
Strain, i.e., deformation causes heat generation. Large deformation increases heat generation. Therefore, in a portion in which strain is large, not only mechanical degradation but also thermal degradation may progress.
Reduction of flexure contributes to durability. In this viewpoint, volume of components, such as clinches and apexes, of bead portions is increased in some cases. However, in this case, a problem arises that the tire becomes heavy and, further, cost is increased.
In general, in a carcass of a tire, a carcass ply is turned up around beads. Thus, the carcass ply has turned-up portions. The turned-up portions contribute to stiffness of the bead portions.
When flexure occurs in the tire, force in a compressing direction acts in the outer side portion in the bead portion, and force in a tensile direction acts in the inner side portion therein. The turned-up portions are disposed in the outer side portions, whereby the turned-up portions are compressed. This compression is likely to cause separation of cords included in the turned-up portions. The loosening may start from the separation of the cords.
From the viewpoint of controlling of stiffness in bead portions, the structure in the portions is variously studied. An example of the study for the structure is disclosed in JP2007-210363 and JP2012-025280.
When a truck runs, deformation and restoration are repeated in the tire. Thus, heat generation occurs in the tire. As described above, large deformation increases heat generation. By the running, a temperature rises in the tire for a truck.
In a case where the truck is parked after running, deformation of the tire due to load persists. In this state, the tire is cooled. Thus, deformation may be fixed. Even if the load becomes zero, restoration from the deformation does not occur. The deformation is referred to as flat spot. In the tire in which the flat spot is generated, vibration occurs during running. The tire is poor in ride comfort.
A main cause of generation of a flat spot is that an apex of a bead is fixed in a deformed state, and a cord of a band contracts due to heat. When volume of rubber of the apex is reduced, improvement may be performed so that the flat spot is reduced. However, this may reduce stiffness of the bead portions. This reduces durability of the tire and ride comfort. Further, when the band is changed from a full band to edge bands to reduce an amount of the cord, improvement can be performed so that the flat spot is reduced. However, reduction in an amount of the cord reduces a force with which the band holds a belt. This may cause “tread separation” in which the surface of the tread is separated. The tire is poor in high-speed durability.
An example of study for reducing generation of flat spot is disclosed in JP2008-168702. In the tire, by positions of the ends of a belt and the ends of a band being adjusted, generation of flat spot is reduced.
A pneumatic tire is mounted on a rim and used. When the tire is mounted on the rim, the bead portions of the tire are fitted to the rim. Air is filled inside the tire. By the air filled thereinside, the bead portions slide outward in the axial direction along a seat surface of the rim. By the sliding, the bead portions come into contact with flanges of the rim, and are disposed at appropriate positions. Thus, the filling with air and disposing the bead portions at appropriate positions relative to the rim is referred to as air-in. The paired bead portions come into contact with the bead flanges, respectively, and mounting of the tire on the rim is then completed. A distance between the paired bead portions prior to the tire being mounted on the rim is increased relative to a distance between the paired rim flanges, whereby the air-in performance can be enhanced.
In a case where a distance between paired bead portions is increased in a tire, when the tire is mounted on a rim, the paired bead portions are moved inward in the axial direction. The bead portions are deformed by the movement. By filling with air, the bead portions are further deformed. In the tire, a tire profile is greatly changed. The tire is mounted to a vehicle, and is under a load. Due to the load, deformation of the bead portions is further increased. In the tire, deformation is large around the bead portions. In particular, in a tire in which a distance between bead portions is increased relative to the tread width or the tire maximum width, strain is high around the bead portions. In the tire, loosening is likely to occur between the beads and a carcass. The tire tends to be poor in durability.
Durability of the tire can be improved by stiffness of the bead portions being improved. By volume of rubber being increased, stiffness of the bead portions may be improved. Further, a carcass structure which has the apparently increased number of carcass plies of the carcass to contribute to stiffness, for example, an HTU structure (highly turned-up structure) may improve stiffness.